This invention relates to a method for effecting vacuum deaeration with improved operational efficiency and more particularly to a method for vacuum deaeration which fits the deprivation of dissolved oxygen in pure water.
In the system using pure water, the pure water is required to possess an extremely low dissolved oxygen concentration for the purpose of preventing various component parts of the system such as, for example, pipes, heat exchangers, and boilers from corrosion or protecting ion-exchange resin against deterioration by oxidation.
Particularly, in recent years, the field of electronic industry specializing in the production of LSI's and super-LSI's has come to feel the need of pure water possessing a finally attainable dissolved oxygen concentration of not more than 10 bbp to ensure prevention of the formation of a native oxide film on semiconductor wafers.
In the methods heretofore available for decreasing the dissolved oxygen content in water, the method for vacuum deaeration using a vacuum degasifier column, the method for heat deaeration using a heat deaerator, the nitrogen bubbling method for depriving water of dissolved oxygen by bubbling nitrogen gas through the water, the method for membrane deaeration which deprives water of dissolved gas by passing the water through a hydrophobic membrane and vacuumizing the outer side of the membrane, and the method for catalytic deaeration which comprises causing dissolved oxygen to react with a reducing agent thereby effecting removal of the oxygen as converted into water, for example, have been renowned.
The method for heat deaeration has the disadvantage that the heat deaerator is difficult to handle and is prone to external contamination; the nitrogen bubbling method has the disadvantage that since the treated water is saturated with nitrogen, the dissolved nitrogen will eventually depart in the form of bubbles from the treated water; the method for membrane deaeration has the disadvantage that the use of the special membrane boosts the running cost of the operation; and the method for catalytic deaeration has the disadvantage that the use of expensive Pd adds to the cost of operation.
The method for vacuum deaeration effects the removal of dissolved oxygen from water by keeping the pressure inside the vacuum degasifier column at a degree of vacuum slightly higher than the vapor pressure at a prescribed water temperature and causing the water meanwhile to be sprayed downwardly onto packings placed in the column. Owing to high economy and freedom from the danger of contamination, this method has been finding popular acceptance. This method, however, is deficient in capacity of deaeration because the lowest attainable dissolved oxygen concentration is rather high, reaching the order of some tends of ppb.
Japanese Patent Application Disclosure (KOKAI) HEI 2(1990)-909,844 and Japanese Utility Model Publication SHO 61(1986)-35,274 disclose methods for effecting gas-liquid contact by injecting an inert gas into the interior of a column and meantime causing a water subjected to treatment to fall down the column interior from above. By these methods, however, the gasliquid contact is carried out invariably under the atmospheric pressure and the treated water is not deaerated.
Japanese Patent Application disclosure (KOKAI) SHO 59(1984)-179,113 discloses a method for deaerating a seal oil by causing an inert gas to leak into a vacuum degasifier column and allowing a flow of the inert gas to contact a flow of the seal oil. This method is aimed at volatilizing such low boiling inflammable gas components as hydrocarbons and H.sub.2 S in the seal oil thereby lowering the flash point of the seal oil and preventing the seal oil from hazards of fire and explosion while storage. The present invention which accomplishes the removal of a dissolved gas present in a minute amount in water is suggested nowhere in this patent application disclosure (KOKAI).